This invention relates generally to vacuum deposition techniques and more particularly to vacuum deposition techniques employing laser beams and is more particularly directed to an improved method and apparatus for adjusting the operating frequency of crystal resonators.
Currently, crystal resonators are adjusted to frequency by placing the semi-finished unit into a vacuum system and evaporating additional plating material (generally gold) onto the electrodes of the unit. The frequency adjusted resonators are then sealed into packages and housings for distribution or use in various electronic circuits. Generally, after being sealed, no further frequency adjustment is possible.
The problem created by this technique is that the frequency of operation may shift between the point the crystal resonator is adjusted, and the point it is sealed. This is particularly true in the typical mass production process where a large quantity of crystal resonators are adjusted and may sit for prolonged periods before being sealed into packages or housings. Generally, the frequency shift is due to exposure to heat or harmful atmospheric conditions (for example, moisture) which may begin to oxidize the plating material.
Some crystal resonator manufacturers adjust the resonator's operating frquency by subtractively trimming the plating material with a laser beam. Typically, the laser power is focused and adjusted to remove a portion of the elctrodes deposited on the crystal. This technique is also prone to subsequent operating frequency shifts since the localized power of the laser beam may change the electrical characteristics (i.e. resistance) of the crystal. Further, the trimmed plating material may migrate to unwanted areas causing crystal contamination. Thus, a need exists to provide a method for accurately adjusting the frequency of a crystal resonator that prevents subsequent frequency shifts and yet is realizable in a mass production process.